1. Field of the Invention
The field of art to which this invention pertains is material severing and, more particularly, it is desired to a method of and apparatus for severing thermoplastic film.
More specifically, such invention relates to a method of and apparatus for engaging the surface portion of a film from a supply roll and moving it to a position in a path of the film whereat such film is transferred to another holding means. The first holding means then returns to its original position where it engages another portion of the film; with the film so engaged by the first and second holding means, it is severed therebetween, using two knives, to form a package member.
2. Decription of the Prior Art
Film severing methods and apparatus of this general type are old. It is known, for example, to move a film, by manual or other means past a film severing device and to sever it while it is held on each side of such severing device.
U.S. Pat. No. 3,748,939 to Feinstein et al, which is typical of the art, shows an apparatus for severing sheet material which includes a transverse web cutting means having a blade that is mounted in a blade carriage which is frictionally affixed to an endless cord and drive means for transversely moving the blade across the sheet and returning it to its starting position. U.S. Pat. No. 3,633,447 to Casida shows apparatus for cutting metal sheets in which cutting elements on a continuous chain are used to cut such metal; again, a commonly used arrangement for cutting or severing sheet material.
And, lastly, U.S. Pat. No. 3,910,008 to Johnson shows the use of a hot cutting wire for severing film as it is held by and between vacuum frames, as a step in making a vacuum package.
It is often difficult to cut or sever a web or sheet of material, particularly thermoplastic film, across its width because of the high stresses as it strikes the edge of the film at the crucial starting phase of such cutting. There is a tendency, for example, for the web to be pulled away from the various devices holding it in position for severing but, more importantly, there is a real possibility that the film will not be cut or severed accurately and cleanly without unnecessary wear and tear on the cutting or severing device. In addition, there may be a tendency for the film to bunch up when struck by the severing knife, at the start of cutting, which further complicates the severing operation.
A problem with the teachings of the art as discussed above, and with the other known art, is their failing to provide or give to the film severing art, a relatively simple means and method of severing a film, without undue cutting or severing stresses, as in the method of the instant invention, by moving a severing starting knife or means into the film near one of its edges and forming a partial cut, followed by the moving, in an opposite direction, of a film completing severing knife or means into this partial cut (without any stress problems occurring) and through the film to sever it.